Re: [RFC/PATCH] cpuset: cpuset irq affinities

Ingo Molnar
2008-02-29T21:32:38Z

* Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> > Could you educate me a little, Peter, on what these irqs are and on
> > the sorts of ways people might want to place them across CPUs?
>
> I'm not sure I know what you're asking. IRQ are hardware notifiers and
> do all kinds of things depending on the hardware. Network cards
> typically use them to notify the CPU of incoming packets. Video cards
> can do vsync notifiers, empty dma buffers, whatnot.

irq affinity masks can basically be thought of as: "these are the CPUs
where external hardware events will trigger certain kernel functions and
cause overhead on those CPUs". An IRQ can have followup effects: softirq
execution, workqueue execution, etc.

so managing the IRQ masks is very meaningful and just as meaningful as
managing the affinity masks of tasks. You can think of "IRQ# 123" as
"special kernel task # 123".

Ingo